Electricity may be distributed in three phases; this requires a separate cable for each phase. Some electricity cables carry up to 400,000 volts and are about 150 mm in diameter. The current and voltage in each cable (or phase) varies sinusoidally with respect to time and the cycles of the three phases are offset from each other by a third of a cycle. Due to the varying current in each cable, each cable has a varying magnetic field around it. Since in normal operation the current cycles in the three cables are offset from each other, the resultant magnetic field produced by the three cables is zero.
However, if there is a failure and a short-circuit between the cables occurs, the current cycles in the three cables synchronise. Since the current cycles are in phase with each other, the magnetic fields produced by the three cables no longer cancel each other out and hence there is a magnetic force between them. This force, which can be as great as 7 tonnes, causes the cables to repel away from each other. If the cables are tied down with a fixing of an insufficient strength, for example, then in the event of a short circuit the cables can break away from the fixing, causing significant damage to the surroundings. It is therefore known to use a cable clamp, otherwise known in the art as a cleat, to secure three phase cables.
One type of cable clamp that is suitable for securing a single cable comprises a lower aluminium cast part having a semi-circular recess, and an upper aluminium cast part, also having a semi-circular recess. The semi-circular recesses may be both approximately the same diameter as that of the cable the clamp is intended for. The upper and lower parts of the clamp are held together by a nut and bolt at either side.
During installation of such a clamp, the lower part is bolted to a support in such a way that a portion of the bolts protrudes upwardly above the surface of the lower part. A portion of cable is then located within the semi-circular recess of the lower part and the upper part of the clamp is placed on top of the cable in such a way that the cable is located within the semi-circular recess of the upper portion. The upwardly protruding portions of the bolts are then used to secure the upper clamp part to the lower clamp part; thus securely clamping the cable.
Features of a cable clamp of this design are as follows: Firstly, since the parts are cast in aluminium they are relatively expensive to manufacture and cannot be produced in high volumes particularly quickly. Secondly, since a portion of the attachment bolts protrudes from the surface of the lower part when it has been attached to a support, it is possible to snag the cable on the bolts, thus damaging the cable. Thirdly, the joint between the upper clamp part and the lower clamp part (including the bolts) creates protrusions which may damage the cable. Fourthly, particularly in situations where access is poor, it can be difficult to install such cable clamps due to the type of fixings required.
In urban areas, three-phase power cables may be provided in underground tunnels. Each of the three cables carries up to approximately 400,000V and has an external diameter of between 100-160 mm as mentioned. Each of the three cables is individually supported from the wall of the tunnel at 8 m intervals. Since the cables can weigh in the region of 40 kg/m it is not appropriate to support them using a clamp as described above. Therefore, another type of clamp is used which includes a saddle portion. This type of clamp comprises a support bracket, a cable-supporting saddle, and a cable-clamping member or strap. The bracket is mounted to a support, the saddle sits within the bracket, the cable is located within the saddle, and the member providing the strap is fitted around the bracket, saddle, and cable so as to hold the cable tight within the assembly. The saddle is longer than the strap in the axial direction of the cable and is curved downwards at either end. The cable fits to the curvature of the saddle and sags in between the positions at which it is supported by clamps. This is to allow for the weight of the cable and its expansion and contraction due to thermal variation.
This type of saddle clamp comprises three individual parts that must be manufactured separately and be fitted together in-situ. The saddle clamp can be quite awkward to assemble, especially where space is an issue. Further, due to the manufacturing processes required for the parts, such as casting, the saddle clamp cannot be produced in large volumes quickly. This has an impact on the cost of the saddle clamp.